Due to the success of the Internet, the Internet Protocol (IP) has become the primary networking protocol. Major concerns of the Internet community are the depletion of global IP address space (IPV4) and the complexity of configuring hosts with global IP addresses for Internet access. To extend the life of current IP address space and provide configureless access, network address translation (NAT) and its extension port address translation (PAT) have been employed.
Network address translation supports connectivity between the Internet and hosts using private addressing schemes. This connectivity provides configureless access to the Internet in that hosts may have independently assigned, globally non-unique addresses that need not be coordinated with the Internet Address Numbering Association (IANA) or other Internet registry. Network address translation pairs up the private addresses to public addresses so that the inside IP addresses appear as legally registered IP addresses on the Internet.
Port address translation allows a number of private network addresses and their ports to be translated to a single network address and its ports. Thus, multiple hosts in a private network may simultaneously access the Internet using a single legally registered IP address. The registered IP address is typically assigned to a router that translates addressing information contained in message headers between the addressing schemes.
A problem with Network and Port Address Translation is that some applications embed addressing information in their message payload data. This embedded addressing information is also to be translated when the packet is crossing the boundary. Unfortunately the translation function does not have the knowledge of the application packet format nor does it know if the packet has embedded addressing information. Therefor it is not possible for the translation function in the border routers to translate such data packets without specific knowledge of such applications and their packet formats. The mechanism to translate such packets is to be implemented in the translation function. Translation functions fail as soon as a new such application is developed or used with the router. Normally vendors of such translation functions in the routers develop new versions to handle the newly discovered applications that embed addressing information in their packets and make new release of the software and update all the affected installed systems. This is time consuming, expensive and cumbersome besides user application downtime.